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Faced with the only possibility of having their workforce, the workers and families emptied that rural England towards the cities, in a totally uncontrolled and desperate way, where hygiene conditions and public health infrastructure were conspicuous by their absence. In large cities, such as London, they just built the strictly necessary to accommodate poorly these workers.

Amidst such a breeding ground, any one of us could imagine that those environments were a proper place for disease and infection. But in the middle of the XIX century, they did not know the same thing that we know now by pure popular wisdom. Specifically, the coronavirus of the time was called cholera, and it was the one that took the most lives in the century in question.

The theory assumed by the population and the health authorities for the spread of cholera was that of impure air or miasma. Germs were hardly known and Louis Pasteur had not yet stated his theory. The reality is that people knew very little about that disease. And John Snow also didn’t know about it ... but he was aware of it. So he set out to investigate the origin of cholera from a strict scientific perspective, hypothesizing and conducting experiments. Was it possible to test the miasmatic theory?

During one of the most dangerous waves of this disease, in 1854, a specific outbreak occurred in the Soho neighbourhood. In the space of three days, 127 people died. Snow went immediately and set about gathering information with the help of an Anglican priest in charge of a clerk in that same neighbourhood, Henry Whitehead.

Almost all of the deaths in the first three days occurred on the same street, Broad Street, which was highly suspicious of believing in the validity of the miasmatic theory. But the Eureka moment surged wan John Snow developed his famous map, marking the location of the infected people, which is known as one of the first epidemiological studies in history.

The close proximity of all those infected to the public fountain in Broad Street, marked in red, prompted Snow and Whitehead to ask the London city council to close it immediately. And they did it.

The map has passed to posterity, and we can assume that John Snow discovered this cholera origin by a fairly simple clustering method, one of the basics today in artificial intelligence. But John Snow went further, and discovered that the water company from which the people drank drew water from the Thames, which at that time was only full of sewage from homes. The relationship between poor water quality and the origin of cholera was becoming clearer.

However, his theory still remained asleep for about 40 more years, and after the cholera wave, the city council reopened the public source on that street.

It was necessary to wait until Pasteur, in 1864, to understand the relationship between germs and fermentation, and especially Robert Koch, in 1876, who demonstrated without a doubt through an experiment with livestock, that a microscopic being could be the responsible for an illness.